The present invention relates to a voltage converter circuit for converting an input voltage and outputting the converted voltage and to a display device equipped with the voltage converter circuit, and more particularly to technologies effective for being applied to a driver circuit of a display device used with a portable device.
A liquid crystal display device of a thin film transistor (TFT) type having switching elements in a pixel area is widely used as a display device of a personal computer and the like. A display device of the TFT type is also used as a display device of a portable terminal device of a mobile phone and the like. A display device used with a portable terminal device is required to be more compact and have lower power consumption performance than a conventional liquid crystal display device.
A problem associated with compactness is a reduced space for mounting a driver circuit of a display device. A narrower peripheral area (narrow frame) than a display area is generally preferred. However, the peripheral area of the display area is an area to be used for mounting a driver circuit. The narrow frame forces the driver circuit to be more compact, and the mount area is restricted to a small space. Furthermore, a high resolution display device has been developed, and as the number of outputs from a driver circuit increases, a contact terminal pitch becomes narrower, posing a problem of lowered contact reliability.
In order to realize a driver circuit capable of being mounted in a smaller area and solving the problem of lowered contact reliability, a so-called driver circuit integrated with a display device has been developed and practically used. This display device is manufactured by forming the driver circuit on the same substrate on which switching elements are formed in a pixel area, by manufacture processes similar to those of forming the switching elements.
A display device of a portable terminal device is required to have low power consumption. It is also required to be driven by a portable power source such as a battery. However, various voltages are required for driving the display device. If a single low voltage of a battery or the like is used, it is necessary to generate voltages for driving the display device from a single power supply voltage by using a booster circuit or the like.
The booster circuit used for this purpose is disclosed, for example, in United States Patent Application Publication 2005/0206441 (JP-A-2005-304285). The disclosed booster circuit has a first transistor, a second transistor, a first capacitor element, a second capacitor element, a diode and an inverter, wherein one electrode of the first transistor is applied with a predetermined potential, an output side of the inverter is connected via the second capacitor element to the gate electrode of the first transistor and to one electrode of the second transistor, an input side of the inverter is connected to the other electrode of the first transistor via the first capacitor element and to the gate electrode of the second transistor, and the diode is connected in a forward direction between the other electrode of the first transistor and the other electrode of the second transistor.